Practical_Antenna_Handbook_0071639586

C h a p t e r 2 : r a d i o - W a v e P r o p a g a t i o n 55
Ionosphere
Signal
refracts
T time for round trip
H 3 108 m/s T
2
Virtual
height
(H)
Earth's surface
Transmitter
Figure 2.31 Finding the virtual height of the ionosphere.
other hand, will depend on the state of the ionosphere 2500 mi away from the site
where f C is measured, so the two indices do not track perfectly.
On average, the best propagation occurs at frequencies just below the MUF. In fact,
the frequency of optimum traffic (FOT) is approximately 85 percent of the MUF. Near the
FOT, signal strengths are optimum because ionospheric absorption is at a minimum
and signal-to-noise ratios (SNRs) are highest because atmospheric noise generally decreases
with increasing frequency.
Lowest Useable Frequency (LUF)
Below a certain frequency, the combination of ionospheric absorption and atmospheric
noise, static, etc., conspire to hinder and even prevent radio communications. The lowest
useable frequency (LUF) is a measure intended to help identify the lower frequency
limit for successful communications.
Unlike the MUF, the LUF is not dependent solely on atmospheric physics. The LUF
of a system can be varied by controlling the signal-to-noise ratio of the transmitterreceiver
path. Although many of the factors that contribute to SNRs are beyond our
control, the effective radiated power (ERP) of the transmitter is one that often can be
changed. For instance, a 2-MHz decrease in LUF is available for every 10-dB increase in
the ERP of the transmitter. In practice, this can be accomplished through the use of